1. Field of the Invention
The present invention relates to the manufacturing field of semiconductor integrated circuits and, more particularly, to a vertical parasitic PNP transistor in a BiCMOS process. The present invention also relates to a manufacturing method of the vertical parasitic PNP transistor in a BiCMOS process.
2. Description of Related Art
Higher and higher cutoff frequency of devices is required for radio frequency (RF) applications. In BiCMOS processes, NPN transistors, particularly Silicon-Germanium heterojunction transistors (SiGe HBT), are good choice for ultra-high frequency devices. Besides, the SiGe process is basically compatible with the silicon process, therefore SiGe HBT has been one of the mainstream ultra-high frequency devices. Under this background, the demands on output devices are correspondingly increasing, for example, the output devices are required to have certain current gain factor and cut-off frequency.
Now, output devices can adopt vertical parasitic PNP transistors. And in the present BiCMOS process, the collector of the vertical parasitic PNP transistor is often picked up through the following way: first forming a buried layer or a well at the bottom of a shallow trench isolation (STI) or shallow trench field oxide in such a way that the buried layer or the well contacts with the collector region of the device which is formed in an active region and picks up the collector region into another active region adjacent to the collector region, and then forming a metal contact in the another active region to pick up the collector (the electrode of the collector region). This way is determined by the characters of the vertical structure of the device. Its disadvantage is the large device area and the large connecting resistance of the collector. Because the collector is picked up through another active region adjacent to the collector region and the another active region needs to be isolated from the collector region by STI or other filed oxides, further reduction of the size of the device is greatly limited.